Duncan Shares Bruno Rossi Prize for Ultra-magnetic Stars

2 January 2004

Will Deliver Prize Lecture on Magnetars Jan. 7 in Atlanta

ATLANTA — It all started when two young physicists, between lectures at Princeton University, began to wonder why radio pulsars are so highly magnetized. Little did Robert Duncan and Christopher Thompson suspect, seventeen years ago, that the magnetism of radio pulsars is feeble compared to the powerful magnetic fields that their work would reveal: fields that alter the very structure of the quantum vacuum. Five years later, they predicted a new class of ultra-magnetic, X-ray luminous, flaring stellar corpses, thousands of times more magnetic than pulsars. This prediction and the eventual detection of what the two theorists called "magnetars" earned them, along with observational X-ray astronomer Dr. Chryssa Kouveliotou, this year’s Bruno Rossi Prize from the American Astronomical Society (AAS).

Duncan, an astrophysicist at The University of Texas at Austin, and Kouveliotou, of the National Space Science and Technology Center (NSSTC) in Huntsville, Ala., will present the Rossi Prize Lecture jointly on January 7 at the 203rd meeting of the AAS in Atlanta. (Thompson, of the Canadian Institute for Theoretical Astrophysics, is unable to attend the meeting.)

Magnetars and radio pulsars are two types of "neutron stars": compact remnants of massive stars that have ended their normal lives in supernova explosions. Stars heavier than the Sun by a factor of ten or more die in supernovae, dispersing most of their material into space. But at the center of a supernova, runaway gravitational collapse squishes material into a dense ball of neutrons about the diameter of a large city, yet more massive than the Sun.

One end-result of this process, the radio pulsar, has been known since the 1960s. Radio pulsars are swiftly-rotating neutron stars that give off radio waves from charged particles streaming above their magnetic poles. Their signals appear to pulsate as their radio beams sweep past Earth, like lighthouse beacons. A typical radio pulsar has a magnetic field that measures about a trillion Gauss. (For comparison, a common refrigerator magnet has a magnetic field of 100 Gauss; and the Sun’s magnetic field can reach 5,000 Gauss within magnetic sunspots.)

Birth of the Magnetar (1987-1998) Duncan and Thompson’s calculations, first done in 1987, predicted a new type of neutron star with a magnetic field that is 1,000 times stronger than a radio pulsar’s. "But for five years we didn’t really understand what these calculations meant," said Duncan. "We were just trying to think of some way to scale down these strong magnetic fields, in order to understand radio pulsar magnetic fields, which are much weaker."

By 1992, the researchers had realized that radio pulsars are only one subclass of neutron stars: those born rotating so slowly that their global magnetic fields are not greatly amplified during the first minute after they form in the cores of supernovae. In other words, radio pulsars are actually weakly magnetized when one considers the range of physical conditions within neutron stars… despite the fact that they have trillion-Gauss magnetic fields. Neutron stars born rotating faster would become "magnetars," with bright X-ray emissions powered by their decaying magnetic fields.

A magnetar, Duncan and Thompson soon realized, is a strange, powerful beast, like a radio pulsar on steroids. Magnetar magnetic fields are strong enough to radically alter fundamental physical processes in their vicinity, splitting photons in two and polarizing the vacuum. These bizarre stars had never been seen, or so most astronomers thought.

There had been a few fleeting, enigmatic observations by space satellites of emissions from astronomical objects that the two scientists thought could be magnetars, but no one had tracked down and studied these sources carefully enough to find telltale magnetar properties. These mysterious objects included the so-called "soft gamma-ray repeaters" (SGRs) and "anomolous X-ray pulsars" (AXPs).

An SGR is a star that repeatedly gives off very intense bursts of "soft," or low-energy, gamma rays. All SGRs found so far lie inside or near the Milky Way. (They are not the sources of the mysterious gamma-ray bursts (GRBs), which have been found to lie far outside our galaxy, near the edges of the known universe.)

An AXP is a neutron star which rotates with period of about 10 seconds, and emits X-rays which seem to pulsate on the rotation period, due to the changing orientation of the star. In the 1990’s, these X-rays were a long-standing mystery: they were powered by some "anomalous" stellar energy source which astronomers did not understand, hence the name. Some AXPs, and some SGRs, are found in young supernova remnants.

Duncan and Thompson argued that both SGRs and AXPs are magnetars. The two spent almost a decade theorizing and hoofing it through scientific meetings trying to convince other scientists that magnetars were real, and that the bursts from SGRs, and the pulsating X-rays from AXPs, were powered by the decay of stupendously-strong magnetic fields.

An alternative picture, favored by many scientists during the 1990’s, involved a disk of material orbiting around a neutron star, somewhat like the rings of Saturn. In this alternative theory, the inner part of the swirling disk gets sucked down onto the neutron star by tremendous gravitational forces, releasing heat and powering observed X-ray and gamma-ray emissions.

"When we first suggested that SGRs and AXPs were magnetically-powered, most astronomers thought that the whole idea was crazy," Duncan said. "It seems funny now, but at the first scientific conference we went to, in 1992, we were allowed three minutes to present our quite elaborate theory. As late as the January 1998 AAS meeting, I was the last person scheduled to talk, for ten minutes, shortly after someone who argued against Einstein’s theory of relativity."

Kouveliotou’s Proof (1998) But by January 1998, Kouveliotou at least was taking the magnetar idea very seriously. She was, in fact, leading an international team of eleven scientists in a concerted effort to check some of the predictions of the model. Using American and Japanese X-ray telescopes borne above Earth’s obscuring atmosphere on satellites, Kouveliotou and her team discovered that SGRs, like AXPs, emit pulses of X-rays even when they are in the non-bursting "quiet" state. Moreover, the X-ray pulse rate was slowing down in the way that matched magnetar predictions. This was widely, but not universally, recognized as a dramatic confirmation of Duncan and Thompson’s theory.

The magnetar model suddenly became the favorite for explaining SGRs, since it also provided an explanation for bright outbursts from SGRs. In the magnetar model, these bright flares are due to instabilities in the magnetic field, much like flares seen on the surface of the Sun, except that extreme magnetism means that magnetar flares are tremendously powerful and intense. Indeed, an August 1998 magnetar flare zapped Earth’s outer atmosphere and significantly affected nighttime radio communications, even though the flaring star was 20,000 light years away.

The AXP Debate (1998-2002) The disk model offered no compelling explanation for the tremendous outbursts from SGRs. But many astrophysicists still favored the disk model as an explanation for the AXPs, since these stars had not shown any bright outbursts. So after 1998, X-ray astronomers who studied AXPs were divided into two warring camps: disks and magnetars. The race was on to find decisive observational evidence that could resolve the debate. This was very difficult because AXPs are exceedingly faint and hard to find among the myriad stars of our Galaxy, if you search for them using any type of radiation except X-rays.

Finally, in 2002, Caltech researchers Brian Kern and Christopher Martin used the Mt. Palomar telescope to show that a nearby AXP gives off a faint optical glow which pulsates on the stellar rotation period. The glow is no brighter than a single, flickering candle at the distance of the Moon. This is much fainter than the disk model predicted; moreover a disk would shine steadily, rather than pulsate. But a diffuse, hot gas of particles trapped in the magnetic field surrounding a magnetar plausibly shines faintly; and this unearthly glow would naturally appear to pulsate as the star rotates, since different views of the star’s magnetic field are presented to Earth as the star turns.

Also in 2002, Victoria Kaspi and Fontis Gavriil of McGill University, working with Peter Woods of NSSTC, showed that AXPs actually do emit bright bursts of soft gamma rays, very much like SGRs. Based on all this new evidence, in January 2004 the disk camp is mostly deserted, and magnetars seem on their way to becoming permanent members of the celestial bestiary.

Magnetars in 2004 Descriptions of magnetars can now be found in dictionaries, encyclopedias and some introductory astronomy textbooks. Perhaps more tellingly, magnetars have become part of the popular culture, appearing in science fiction stories and novels. ‘Magnetar Games’ is a popular video-game company, and ‘Magnetar Technologies’ makes ‘magnetar’ magnetic brakes for amusement-park rides, among other commercial products. There are at least two rock bands named ‘Magnetar.’

"I bought a Magnetar CD on the internet," Duncan said. "It is probably the worst music I have ever heard."

The Rossi Prize is named for Dr. Bruno Rossi, who was a pioneer of X-ray astronomy. It is awarded annually, and internationally, for outstanding contributions to high-energy astrophysics. Duncan is the first Texas scientist to receive the Prize. Only three previous Rossi Prizes have been given to theoretical astrophysicists, since the award was endowed 19 years ago.

A Hubble Space Telescope image of a supernova remnant associated with a magnetar can be found here. A caption follows:

BIRTHPLACE OF A MAGNETAR: 5,000 years ago, a massive star died violently within an irregular clump of stars which orbits our galaxy (the "Large Magellanic Cloud"). This ancient supernova left behind the expanding, glowing remnant of hot gas shown in this Hubble Space Telescope photo. The supernova also evidently left behind a magnetar --a compact, ultra-magnetic stellar corpse, powered by magnetic energy -- which is nearly invisible in ordinary, optical light, but glows brightly in X-rays. It is displaced from the center of the supernova remnant, suggesting that the neutron star received a "kick" at birth of about 1000 kilometers/second and subsequently drifted downward across the sky. According to Duncan and Thompson, this kick was probably induced by "neutrino magnetic starspots" in the newborn magnetar: a phenomenon analogous to sunspots. The star emitted a tremendous flare which reached Earth on March 5, 1979, and which was the brightest flux of gamma-rays detected from outside our Solar System until a second magnetar flare blitzed the Earth in 1998. This "March 5th event" provided astronomers with crucial evidence for the existence of magnetars.